This thesis is based on the results of my PhD studies at Palacky University in Olomouc. The research deals with the task of improvement of existing quantum communication protocols with multimode entangled states. Firstly, we consider the mode-multiplexing in entanglement distribution and quantum key distribution, in this case each mode is used to carry separate quantum signal, hence it should be handled and measured separately. The other issue we study is the use of multimode bright states of light in quantum communication, in this case the modes are not discriminated in the measurement, the protocol does not use them for multiplexing, but instead multiple modes make the signal brighter and easier to work with in experimental implementation. We study multiplexing of the entangled states of light used in quantum communication and, in particular, in quantum key distribution. Mode-multiplexing allows to improve performance and increase capacities of quantum communication protocols. Unfortunately while improving protocols capacities, the multi-mode structure of quantum states can also introduces new imperfections, that are not present in single-mode implementations of the protocols. Our work is devoted to the study of some of these imperfections. The main focus of ours is the intramode cross talk and the ways to compensate it. In the process of generation, distribution and measurement the modes can get coupled to each other due to photon exchange between them, i.e. they experience cross talk. We theoretically study deteriorating effects of the cross talk on entanglement and the secure key in a simplified 4-mode model and suggest methods that mitigate negative influence of the cross talk. The approaches we suggest can be both passive (optimization of the state during its preparation) or active (introducing network of optical elements that compensate for the cross talk). We then proceed to apply one of the active compensation methods to improve the source of frequency-multiplexed entangled light with strong coupling between the modes. We model the quantum key distribution protocol using the frequency-mode multiplexed entangled state produced experimentally by the group from Laboratoire Kastler Brossel. We show that after cross talk compensation the secret key rate of the protocol increases significantly, confirming viability of the proposed cross talk compensation method. Lastly, we study applicability of multimode bright states for quantum key distribution. The imperfect matching of the multi-mode signal with the phase reference beam during the measurement introduces noise to the signal, negatively affecting the quantum key distribution protocol performance. We demonstrate with the experimental data from the group from Max Planck Institute for the Science of Light, that the noise introduced by unmatched modes can be suppressed by the increase of the reference beam power, hence restoring the secret key.This thesis is based on the results of my PhD studies at Palacky University in Olomouc. The research deals with the task of improvement of existing quantum communication protocols with multimode entangled states. Firstly, we consider the mode-multiplexing in entanglement distribution and quantum key distribution, in this case each mode is used to carry separate quantum signal, hence it should be handled and measured separately. The other issue we study is the use of multimode bright states of light in quantum communication, in this case the modes are not discriminated in the measurement, the protocol does not use them for multiplexing, but instead multiple modes make the signal brighter and easier to work with in experimental implementation. We study multiplexing of the entangled states of light used in quantum communication and, in particular, in quantum key distribution. Mode-multiplexing allows to improve performance and increase capacities of quantum communication protocols. Unfortunately while improving protocols capacities, the multi-mode structure of quantum states can also introduces new imperfections, that are not present in single-mode implementations of the protocols. Our work is devoted to the study of some of these imperfections. The main focus of ours is the intramode cross talk and the ways to compensate it. In the process of generation, distribution and measurement the modes can get coupled to each other due to photon exchange between them, i.e. they experience cross talk. We theoretically study deteriorating effects of the cross talk on entanglement and the secure key in a simplified 4-mode model and suggest methods that mitigate negative influence of the cross talk. The approaches we suggest can be both passive (optimization of the state during its preparation) or active (introducing network of optical elements that compensate for the cross talk). We then proceed to apply one of the active compensation methods to improve the source of frequency-multiplexed entangled light with strong coupling between the modes. We model the quantum key distribution protocol using the frequency-mode multiplexed entangled state produced experimentally by the group from Laboratoire Kastler Brossel. We show that after cross talk compensation the secret key rate of the protocol increases significantly, confirming viability of the proposed cross talk compensation method. Lastly, we study applicability of multimode bright states for quantum key distribution. The imperfect matching of the multi-mode signal with the phase reference beam during the measurement introduces noise to the signal, negatively affecting the quantum key distribution protocol performance. We demonstrate with the experimental data from the group from Max Planck Institute for the Science of Light, that the noise introduced by unmatched modes can be suppressed by the increase of the reference beam power, hence restoring the secret key.